FIG. 1 is a side view of part of a fastener assembly passing through a composite panel 1. The assembly comprises an externally threaded bolt 2, an internally threaded nut 3, and a washer 4. In the event of a lightning strike hitting the composite panel 1 and attaching to the fastener, sparking, plasma or out-gassing may occur at the locations 5 shown in FIG. 1.
A known method of providing spark suppression is described in WO 2012/107741. A volume of gas is enclosed by a cap around the fastener. The gas provides spark suppression for arcing that may occur between the composite structure and the metal fastener during any lightning strike.
WO 2012/107741 describes a two-piece construction comprising an outer cap member and an inner cap member. In one example the required quantity of sealant is measured into the base of the outer cap member and the inner cap member is then either partially nested within the outer cap and placed over one end of the fastener. By exerting hand pressure to urge the outer cap over the inner cap, towards the structure, the sealant is forced to flow between the inner and outer caps and exit from around the base of the outer cap over the surface of the structure.
Ideally, the sealant is distributed without voids between the inner and outer cap members, and a portion of sealant collects at the base of the outer cap member, to form a bead of sealant around the circumference of the cap, sufficient to adhere the cap to the structure when the sealant cures.
The installation force increases approximately proportional to the cross sectional area and surface area of the space between the inner and outer caps and there is no mechanical advantage. On larger installations, e.g. fasteners larger than approximately ¾ diameter, an unreasonably high installation force is required to install the nut cap by hand.